You can upgrade from WSUS 2.0 to WSUS 3.0 for installations with the Frontend and FullInstall values. For the Backend value, you would uninstall WSUS, leaving the database behind. This database will be upgraded when the front-end WSUS server that points to the database is upgraded.

For WSUS 3.0, there is no Windows Installer product key, but there are two registry values: InstallType supports only two installation types: 1 = install or 2 = console-only install. VersionString is a string of the form Major.Minor.Build.Revision.